Explanation:

In Salesforce B2C Commerce, the cartridge path order determines the priority for file resolution — such as:

controllers
ISML templates
scripts
pipelines

When the same file exists in multiple cartridges, the first occurrence in the cartridge path is used.
This is especially important when overriding templates or logic for specific sites.

🧠 Scenario Recap:
You’re using a multi-site realm.
You want the new site to use a different accountlanding.isml template, but reuse the same business logic (AccountController.js).
You have a new cartridge app_newsite with only the overridden template.

Existing cartridge path:
int_cybersource:int_paypal:app_storefront

✅ What You Need to Do:
You want only the new site to use the overridden template in app_newsite, and fall back to app_storefront for all shared logic.

To achieve this:
You must place app_newsite before app_storefront in the cartridge path.
This ensures the system will find and render accountlanding.isml from app_newsite before falling back to the one in app_storefront.

✔️ Modified cartridge path:
int_cybersource:int_paypal:app_newsite:app_storefront

❌ Why Other Options Are Incorrect:

B. app_storefront before int_cybersource
Not relevant — no conflict with integration cartridges here

C. Include only app_newsite
This would remove all core functionality from app_storefront (bad for logic reuse)

D. app_newsite after app_storefront
The template in app_storefront would override the one in app_newsite, making the override ineffective

📘 Salesforce Reference:
Cartridge Path Resolution Guide
Cartridge Overriding Best Practices